I agree with you there. I never said myBB was a bad software. And I always keep myBB in the equation as its a really nice software now. Im an active admin on both a mybb and SMF forum.
1 - update your forum to SMF 1.1.7. There are some serious security issues that the 1.1.6 - 1.1.7 update fixes. 2 - Install the ReCaptcha modification - http://custom.simplemachines.org/mods/index.php?mod=1044 You will need an account at recaptcha.com, but its free.